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o•per•cu•lum

Pronunciation: (ō-pûr'kyu-lum),[key]—n.,—pl.-laPronunciation: (-lu),[key]-lums.1. Bot.,Zool.a part or organ serving as a lid or cover, as a covering flap on a seed vessel.
2. Zool.a. the gill cover of fishes and amphibians. See diag. under fish.b. (in many gastropods) a horny plate that closes the opening of the shell when the animal is retracted.